1. Field of Invention
The present invention relates to a system for purifying an exhaust gas and an exhaust system having the same. More particularly, the present invention relates to a system for purifying an exhaust gas and an exhaust system having the same which prevents degradation of a selective reduction catalyst by lowering a regeneration temperature of a particulate filter on which the selective reduction catalyst is coated.
2. Description of Related Art
Generally, exhaust gas flowing out through an exhaust manifold from an engine is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe.
A selective catalytic reduction (SCR) catalyst is one type of such a catalytic converter.
Reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) react better with nitrogen oxides than with oxygen in the SCR catalyst.
In addition, a particulate filter is mounted on the exhaust pipe and traps particulate matters (PM or soot) contained in the exhaust gas. If excess soot is trapped in the particulate filter, it is difficult for the exhaust gas to pass the particulate filter and a pressure of the exhaust gas increases. If the pressure of the exhaust gas is high, engine performance may be deteriorated and the particulate filter may be damaged. Therefore, if an amount of the soot trapped in the particulate filter is larger than or equal to a predetermined amount, a temperature of the exhaust gas is raised so as to burn the soot trapped in the particulate filter. Such a process is called a regeneration of the particulate filter.
A conventional system for purifying an exhaust gas includes an oxidizing catalyst for purifying carbon monoxide and hydrocarbon contained in the exhaust gas, a particulate filter for filtering particulate matters (or soot) contained in the exhaust gas, and a SCR catalyst for purifying nitrogen oxides contained in the exhaust gas. In addition, the conventional system for purifying the exhaust gas further includes a first injector for injecting a reducing agent to the SCR catalyst and a second injector for injecting a fuel when regenerating the particulate filter. Therefore, a small vehicle lacks a space where the system for purifying the exhaust gas is mounted.
Meanwhile, the SCR catalyst is mounted at a downstream of the particulate filter according to the conventional system for purifying the exhaust gas. In order to regenerate the particulate filter, an inlet temperature of the particulate filter is raised to at least 600° C. In this case, an inner temperature of the particulate filter is raised to a temperature higher than 800° C. as shown in FIG. 4. A degradation temperature is different according to types of the SCR catalyst, but degradation of the SCR catalyst proceeds quickly if a temperature of the SCR catalyst is higher than 800° C. Therefore, the particulate filter and the SCR catalyst are mounted with a predetermined distance. Accordingly, the space where the system for purifying the exhaust gas is mounted is further insufficient in the small vehicle.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.